BEIRUT — An increasingly effective Syrian rebel force has been gaining ground in recent weeks, stepping up its attacks on government troops and expanding the area under its control even as world attention has been focused on pressuring Syrian President Bashar al-Assad to comply with a U.N. cease-fire.

The loosely organized Free Syrian Army now acknowledges that it is also no longer observing the truce, although rebel commanders insist they are launching attacks only to defend civilians in the wake of concerns generated by two recent massacres in which most of the 186 victims were women and children.

The rebels say they are acquiring access to ammunition and funding that had been in short supply a few months ago, streamlining their structures to improve coordination and steadily eroding the government’s capacity to control large swaths of the country.

On Saturday, two neighborhoods in the central city of Homs that have come under the sway of the Free Syrian Army were bombarded by government forces in a further sign that the chances of imposing a cease-fire soon remain slim.

Residents cowered in basements as shells slammed repeatedly into the city, sending clouds of smoke billowing into the sky, according to videos posted online.

The increased activity comes as an international effort to aid the Free Syrian Army quietly gathers pace. Although the rebels insist they are still not being helped by foreign countries, they say they now have ample access to money from Syrian opposition figures and organizations outside the country and are using it to purchase supplies on a revived black market inside Syria.

U.S. officials and Syrian opposition figures said last month that a discreet effort by Arab gulf states to channel funds and weapons to the rebels — an undertaking the United States is helping to coordinate — had begun to gear up. The rebels’ apparent progress suggests that the assistance is having an effect, driving the dynamic toward a deepening conflict and perhaps intensifying pressure on the government to make concessions, even as world powers continue to rule out military intervention, analysts say.

The lightly armed rebels, equipped mostly with assault rifles, rocket-propelled grenades and some mortars, are still a long way from being able to inflict defeat on the superior Syrian army. They are also far from achieving the capabilities of Libya’s rebels, who quickly seized control of a large chunk of territory and a sizable arsenal.

The Syrian rebels “are never going to be capable of driving on Damascus and driving out the regime,” said Jeffrey White, a former Defense Intelligence Agency official who is now with the Washington Institute for Near East Policy. But, he said, with their repeated hit-and-run attacks on Syrian outposts and convoys, it is clear that “they’re increasing pressure on the regime, increasing attrition and increasing defections.”

“They’re looking stronger to me, and better,” White said. “People have been calling them a ragtag force, and I just don’t think that’s accurate anymore. I would describe them as an increasingly capable guerrilla force.”

The rebels are also gaining confidence.

“Every day we control more territory, every day we have more defections, and we are having better organization in our ranks,” said Maj. Sami al-Kurdi, a spokesman for the Homs Military Council, one of the new military structures that are being established around the country. “The regime now controls only the territory under its tanks, and the evidence is that they don’t dare step out of their tanks.”

Growing effectiveness

Verifying such claims is difficult because the Syrian government restricts access to journalists. But military experts who monitor the evolving conflict say they have detected a significant uptick in the number of videos of rebel attacks posted online.

The rebels also say they are inflicting heavy casualties on government troops, and though confirming that is impossible, the official Syrian Arab News Agency has recently reported a growing number of deaths. In the past 10 days, 234 members of the security forces killed in the violence have been buried at honorary military funerals, according to a tally of the reports — a record 57 of them Saturday.

The intensified fighting of the past two weeks offers further evidence of the rebels’ growing strength. The Free Syrian Army said it overran three government positions in the northern province of Idlib last week, and there have been battles in areas previously untouched by the violence, such as the rural area north of Aleppo.

The government is meanwhile escalating its use of force in response to the rebels’ gains. Saturday’s attacks against the Homs neighborhoods of Khaldiyeh and Qusoor were preceded by a sustained bombardment of the southwestern city of Daraa. A total of 53 people were killed nationwide in government attacks, according to the Local Coordination Committees, an opposition group.

But after driving rebels from strongholds in the Baba Amr district of Homs and several Idlib towns in March, the government has since seemed unable to press home the advantage. Repeated efforts to dislodge the Free Syrian Army from the provincial Homs towns of Rastan and Qusair have failed, and a major offensive launched last week against a previously unnoticed rebel stronghold in the town of Haffa, northeast of Latakia, faltered despite intense shelling and the deployment of combat helicopters.

The government still has the capacity to use overwhelming force against rebel strongholds, but it “can’t bring force to bear everywhere at the same time,” said Amr al-Azm, a professor of history at Shawnee State University in Ohio who is active in the Syrian opposition.

“They can go in and suppress an area, but as soon as they leave it, it is out of their control,” he said.

The result is that many parts of northern and central Syria have effectively fallen under the sway of the rebels, said Joseph Holliday, a researcher who tracks Free Syrian Army activity for the Washington-based Institute for the Study of War.

“They’re making it really hard for the regime to move around, for them to get out of their little checkpoints that they’re barricaded into,” said Holliday, a veteran of the Iraq war who compared the predicament of Syrian forces to that of U.S. troops in Iraq during the height of the fighting there. “We were pinned down. We couldn’t get around,” he said.

Signs of cohesion

Rebel commanders credit the growing demoralization of the regular army, in part, for their performance. Defections continue at a steady rate and troops are weary after nearly 15 months of continuous deployment in the effort to suppress the rebellion, they say.

And though they deny receiving foreign help, the rebels say the money they are receiving now is enabling them to buy more weapons on the thriving black market.

“We are getting weapons from dealers who are very active in Syria now,” said Col. Aref Hammoud, a spokesman for the Free Syrian Army in southern Turkey. Rebels are also seizing more weapons in attacks on government forces, he said.

In some areas, such as Hama, Free Syrian Army commanders have started to pay fighters stipends of $50 to $100 a month, in apparent fulfillment of a pledge made at a Friends of Syria conference in April.

In other places, commanders say they are receiving no help, a reminder that the Free Syrian Army is still a highly fragmented force, a concept more than a structure, whose leadership, based in a refugee camp in Turkey, exerts only scant control over the many small rebel bands that have sprung up around the country.

Holliday says he has counted 300 armed groups operating in Syria, though he suspects the number might be closer to 100 once name changes and overlapping chains of command are taken into account.

An effort to organize many of those groups into regional military councils does appear to have instilled a greater sense of cohesion, he said.

“It’s the most encouraging organizational effort I have seen,” he said. “But there’s still a lot of work to do.”